Human cytomegalovirus (HCMV) is a ubiquitous agent in human populations. Infections are generally asymptomatic, but there can be serious medical sequelae in immunocompromised individuals and in congenitally infected newborns. In immunocompromised individuals, HCMV infection can result in interstitial pneumonia, retinitis progressing to blindness and disseminated infection. Infections in newborns can be severely damaging, with multiple organ involvement including the central nervous system and may also result in auditory damage. The mechanisms of pathogenesis are not understood, although it is believed that host factors, such as cellular and/or humoral immune responses might be involved. See, Alford and Britt, "The Human Herpesviruses", eds Roizman, B., R. J. Whitley and C. Lopez, Raven Press, New York, 1993, pp 227-55. It has also been speculated that genetic variability (either structural or antigenic or both) among different strains of HCMV could be responsible for the variance in clinical manifestations observed. Pritchett, J. Virol. 36:152-61(1980); Lehner, J. Clin. Microbiol. 29:2494-2502(1991); Fries, J. Infect. Dis. 169:769-74(1994).
Considerable attention has been focused recently on the analysis of strain variation among HCMV isolates. Some twenty different HCMV strains have been isolated and differentiated by restriction analysis of PCR amplified DNA fragments. Chou, J. Infect. Dis. 162:738-42(1990).
One strain, the Towne strain, has been developed into a live, attenuated vaccine and administered with some success in renal transplant patients. See Quinnan, Annals of Int. Med. 101:478-83(1984); Plotkin, Lancet 1:528-30(1984). However, Towne strain vaccines who were directly challenged by low-passaged Toledo strain wild-type virus in one study were found to resist challenge doses of only 10 plaque-forming units (pfu) or less. Plotkin, J. Infect. Dis. 159:860-65(1989). Therefore, it appears the Towne strain may be overly attenuated, i.e., genetically modified so extensively resulting from serial passage in cell culture that it has lost significant immunogenicity presumably due to the loss of genetic information during the cell passage. Advantageously however, the Towne strain has never been shown to reactivate.
DNA sequence heterogeneity between the Towne strain and another strain of HCMV, AD169, has been found. Pritchett, J. Virol. 36:152-61(1980). (A restriction map of the AD 169 HCMV genome is disclosed in U.S. Pat. No. 4,762,780.) Variation in the DNA content among other isolated strains of HCMV has also been detected. Huang, Yale J. Biol. and Med. 49:29-43(1976). Cleavage patterns of restriction enzyme digests of HCMV DNA of various strains has been analyzed. Kilpatrick, J. Virol. 18:1095-1105(1976); LaFemina, "Structural Organization of the DNA Molecules from Human Cytomegalovirus" in Animal Virus Genetics, eds. Field, B N and R. Jaenish, Academic Press, NY (1980); Chandler, J. Gen. Virol. 67:2179-92(1986); Zaia, J. Clin. Microbiol. 28:2602-07(1990). However, although the gross structural organization of the HCMV genome has been determined and strain-to-strain restriction site polymorphism mapped for many of the strains, strain-to-strain differences in the DNA sequences of the HCMV genome have not been determined. Only partial sequences have been deduced and compared. For example, the DNA and amino acid sequences of the envelope glycoprotein B [gpUL55(gB)] of both Towne and AD169 strains have been deduced, see Spaete, Virology 167:207-25(1988), and compared with various clinical isolates, see Chou, J. Infect. Dis. 163:1229-34(1991), to identify conserved regions and regions of variability. In addition, DNA sequence analysis of certain regions of the gp58/116 gene [gpUL55(gB)], the IMP gene and the IE-1/2 enhancer/promoter has been accomplished. Lehner, J. Clin. Microbiol. 29:2494-2502(1991).
Whereas the complete DNA sequence of the AD169 strain of HCMV has been deduced, (EMBL Accession No. X17403), the complete DNA sequence of the Towne strain has not to our knowledge been deduced. However, it has been speculated that AD169 and another laboratory strain, Davis, are missing two to four kilobase pairs (kb) of DNA sequence compared to the Towne strain at the extreme internal portions of both L repeats. LeFemina, supra, at 52-53.
The public health impact of HCMV infections has not been well controlled by current treatment strategies or available antiviral chemotherapies. Preventative vaccine strategies are likely to prove efficacious because of the observations that seropositive renal allograft recipients are protected from severe HCMV disease and maternal immunity protects the fetus from disease after intrauterine infection. Marshall and Plotkin, "Cytomegalovirus Vaccines" in The Human Herpesviruses, eds Roizman, B., R. J. Whitley and C. Lopez, Raven Press, New York, 1993, pps 381-95. However, an additional obstacle to the development of a vaccine for HCMV is the lack of an animal model system that can be used to test the safety and efficacy of vaccine candidates.
There remains a need in the art for efficacious vaccines for the prophylactic treatment of HCMV in humans.